1. Field of the Invention
The present invention relates generally to a method of polishing silicon wafer. More specifically, it relates to a method of polishing a silicon wafer, by which contamination by a metal, such as copper, nickel, or the like, is prevented while the silicon wafer is being subjected to a mirror-polishing process.
2. Description of Related Art
Usually, a silicon wafer is manufactured by slicing a single-crystal ingot into a wafer, and thereafter applying to the wafer the predetermined steps of chamfering, lapping, etching, mirror-polishing, and cleaning.
At the mirror-polishing step (hereinafter, the mirror-polishing step is occasionally referred to simply as xe2x80x9cpolishing stepxe2x80x9d), in general, a silica-containing abrasive agent, which is alkaline aqueous solution containing fine (i.e., very small) silica (i.e., SiO2) abrasive grains dispersed therein, (referred to as xe2x80x9calkaline silica-containing abrasive agentxe2x80x9d) is used. In case of the alkaline silica-containing abrasive agent, the silicon wafer is polished by means of a chemical etching action of the alkaline aqueous solution and by means of a mechanical action of the silica.
However, commercially available alkaline silica-containing abrasive agents usually contain an impurity of metal (hereinafeter, referred to as xe2x80x9cmetallic impurityxe2x80x9d). Typical examples of the metallic impurity include copper, nickel, chromium and iron.
Accordingly, when the silicon wafer is polished by such a commercially available alkaline silica-containing abrasive agent in which the metallic impurities are present, among the metallic impurities, metal ions, such as copper ion, nickel ion, chromium ion, iron ion, diffuse in a deep interior of the wafer in the course of polishing step, and cannot be easily removed even at the next cleaning step, thereby deteriorating a performance of the semiconductor device manufactured from the wafer, as is known in the art.
As a measure to prevent the metallic contamination, it is conceivable to use a high-purity silica-containing abrasive agent which does not contain such metallic impurities. However, a commercially available high-purity silica-containing abrasive agent is so expensive as to increase the manufacturing cost, so that it cannot be employed for the actual production. Further, even when the high-purity silica-containing abrasive agent is used, contamination caused by the apparatus cannot be avoided, so that the high-purity silica-containing agent, in some cases, fails to exhibit its sufficient effect.
Another abrasive agent obtained by adding several percent by weight of amine to the alkaline silica-containing abrasive agent (referred to as xe2x80x9camine-added silica-containing abrasive agentxe2x80x9d) is often used to increase the polishing speed at the time of polishing the silicon wafer. In case of the amine-added silica-containing abrasive agent, it is known that, although the polishing speed is increased, the metallic contamination to the silicon wafer is rather accelerated, so that it is difficult to obtain a high-purity silicon wafer.
To solve this problem of the amine-added silica-containing abrasive agent, Japanese application laid-open No. 11-186202 shows a recognition that amine is regarded as a cause to accelerate the metallic contamination and discloses a silica-containing abrasive agent which does not contain amine, more particularly, an abrasive agent obtained by adding an oxoacid compound to the silica-containing abrasive agent (referred to as xe2x80x9coxoacid compound-added silica-containing abrasive agentxe2x80x9d).
However, according to the inventors"" study, employing the oxoacid compound-added silica-containing abrasive agent in polishing the silicon wafer neither increases the polishing speed nor produces a sufficient effect in reducing the metallic contamination.
In view of the above-mentioned problems in the conventional art, the inventors have studied various types of abrasive agents to find an optimum abrasive agent for a silicon wafer which can prevent or suppress the metallic contamination at the polishing step while maintaining the polishing speed. As a result, they have found that prevention or suppression of the metallic contamination can be realized by adding alkali sulfide or alkali hydrogensulfide to a silica-containing abrasive agent. Also, they have found that prevention or suppression of the metallic contamination can be realized by adding a chelate agent to a silica-containing abrasive agent.
It is therefore an object of the present invention to provide a method of polishing the silicon wafer using an abrasive agent which can efficiently prevent or suppress metallic contamination to the silicon wafer at the mirror-polishing step so as to prevent deterioration of the quality of the silicon wafer.
To achieve the above object, a first aspect of the present invention provides a method of polishing a silicon wafer using an abrasive agent comprising: silica as a principal ingredient; and an ingredient which is selected from alkali sulfide, alkali hydrogensulfide, and the mixture thereof.
The alkali sulfide and the alkali hydrogensulfide are preferably ammonium sulfide and ammonium hydrogensulfide, respectively.
A second aspect of the present invention provides a method of polishing a silicon wafer using an abrasive agent comprising: silica as a principal ingredient; and a chelate agent which contains at least xcex1-benzoinoxime, diethyldithiocarbamic acid, cupferron, xanthogenic acid, neocupferron, beryllon II, xcex2-quinolinol, 1,1,1-trifluoro-3(2-thenoyl)acetone, dimethylglyoxime, and 1-(2-pyridylazo)-2-naphthol.
The chelate agent may be contained in the form of a derivative thereof or a salt thereof.
Owing to the constitution of the first aspect of the present invention in which the abrasive agent containing alkali sulfide or alkali hydrogensulfide is used in the method of polishing a silicon wafer, the copper ion and the sulfer ion present in the abrasive agent result in the formation and precipitation of copper sulfide slightly soluble in water so as to decrease the concentration of the copper ion in the abrasive agent, whereby the contamination by copper (referred to xe2x80x9ccopper contaminationxe2x80x9d) to the silicon wafer is reduced.
Owing to the constitution of the second aspect of the present invention in which the abrasive agent containing the chelate agent is used in the method of polishing a silicon wafer, the copper ion present in the abrasive agent is trapped by the chelate agent so as to suppress the adsorption of the copper ion onto the silicon wafer, whereby the copper contamination to the silicon wafer is reduced.
Other features, objects and advantages of the present invention will become apparent from the following description of the preferred embodiment with reference to the drawings.